Metal-oxide-semiconductor (MOS) transistor is one of the most important component in modern integrated circuits (ICs). The basic structure of a MOS transistor includes a semiconductor substrate, a gate structure on the surface of the semiconductor substrate, a source region in the semiconductor substrate on one side of the gate structure, and a drain region in the semiconductor substrate on the other side of the gate structure. To operate the MOS transistor, a switching signal may be generated by applying a voltage across the gate structure to control the current in a channel under the gate structure.
However, the performance of existing MOS transistors still needs to be improved. The disclosed semiconductor devices and fabrication methods thereof are directed to solve one or more problems set forth above and other problems in the art.